The present invention relates to pen plotters and, more particularly, in a carriage coupling drive system for a pen plotter including a moving carriage slidably mounted on a shaft between the ends of and carrying a U-shaped pen-carrying yoke in combination therewith for sliding motion along the shaft and rotary motion about the shaft, to the improved low-friction, low-play bearing system comprising, a pair of annular ball-bearing thrust bearings disposed over the shaft between respective ends of the U-shaped yoke and the moving carriage; and, annular bias spring means disposed over the shaft for creating a linear expansion force resisting play between the moving carriage and the U-shaped yoke.
Pen plotters are well known in the art. As depicted in FIG. 1, a typical prior art pen plotter includes a carriage coupling drive system, generally indicated as 10, wherein a moving carriage 12 is slidably mounted on a shaft 14 between the ends of and carrying a U-shaped pen-carrying yoke 16 in combination therewith for sliding motion along the shaft 14 and rotary motion about the shaft 14. The carriage 12 is moved laterally back and forth as indicated by the arrows 18 by means of a drive tape 20 attached thereto. The tape 20 is driven by a motor drive (not shown) to create one axis of a plot. The pen-carrying yoke 16, of course, moves with the carriage 12 carrying a pen 22 in jaws 24 which are formed into the yoke 16. The pen 22 creates the actual lines on the plot by being raised and lowered through rotation of the yoke 16 about the shaft 14 as indicated by the arrows 26. In the raised position, the pen 22 can be moved from place to place laterally along the shaft 14 without contacting the drawing paper (not shown) being moved thereunder to create the other axis of the plot. In the lowered position, the pen 22 contacts the paper so that the combined movement of the pen 22 and the paper causes the pen 22 to draw line vectors along the two axes of the plot.
Prior art carriage coupling drive systems such as that indicated as 10 in FIG. 1 have accomplished the pen-up and pen-down operations in what can be referred to as a bangbang mode; that is, an electric solenoid is mechanically connected to drive the pen 22 down (i.e. rotate the yoke 16 about the shaft 14) against the bias of a spring (not shown) which raises the pen 22 when the solenoid is de-energized. Thus, the pen 22 (and yoke 16) has two extreme position - full up and full down. In such an operational mode, a bearing system such as that shown in FIG. 2 is quite adequate. The ends 28 of the U-shaped yoke 16 are fitted with sleeve bearings 30 fitted into bores 32 through the ends 28 provided for the purpose. Another sleeve bearing 34 is disposed in a longitudinal bore 36 through the carriage 12. The sleeve bearing 34 is of a length to be a slide fit between the two sleeve bearings 30. As thus configured, the bearings can be operated in a dry condition while affording reasonably acceptable levels of friction and resistance to movement in the longitudinal and rotary directions and resisting longitudinal play between the parts which could produce errors in the plot being created. It should be understood that the example of FIG. 2 is representative only and that prior art plotter carriage drive systems have a variety of configurations.
The foregoing generalities notwithstanding, one aspect of prior art carriage drive system design that has remained a problem is the tradeoff between friction and factors such as line quality and plotter throughput. Typically, the design of prior art systems has been line quality driven with the end result being a reduction in plotter throughput. The explanation for this is simple. If the longitudinal clearances are made tight enough to reduce slop, the friction is too high and parts may bind up in the presence of high temperatures causing the parts to expand. Thus, it is more typical in the prior art to provide a clearance fit and externally spring bias the moving carriage 12 towards one side of the yoke 16 or the other. The carriage drive system as thus configured has the required friction/slop features; but, must be decelerated and allowed to stabilize before plotting can begin. Thus, the plotter throughput is diminished. If the carriage drive system were moved rapidly to a new point, stopped, and plotting begun immediately before longitudinal stabilization of the moving carriage 12 relative to the yoke 16 had taken place, the line quality, of course, would suffer.
Wherefore, it is the object of the present invention to provide an improved carriage coupling drive system for a pen plotter having no longitudinal play between the components while having very low friction with respect to rotary motion between the same components.
It is another object of the present invention to provide an improved carriage coupling drive system for a pen plotter which does not require deceleration to allow the components time to longitudinally stabilize before plotting.
Other objects and benefits of the present invention will become apparent from the description which follows hereinafter taken in combination with the drawing figures which accompany it.